Re: [PATCH 0/4] Inline helpers into Rust without full LTO

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 07, 2026 at 01:23:38PM +0100, Andreas Hindborg wrote:
> Hi Alice,
> 
> Alice Ryhl <aliceryhl@google.com> writes:
> 
> > Currently the only way for Rust code to call a static inline function is
> > to go through a helper in rust/helpers/. This introduces performance
> > costs due to additional function calls and also clutters backtraces and
> > flame graphs with helper symbols.
> >
> > To get rid of these helper symbols, provide functionality to inline
> > helpers into Rust using llvm-link. This option complements full LTO, by
> > being much cheaper and avoiding incompatibility with BTF.
> >
> > I ran a microbenchmark showing the benefit of this. All the benchmark
> > does is call refcount_inc() in a loop. This was chosen since refcounting
> > is quite hot in Binder. The results are that Rust spends 6.35 ns per
> > call vs 5.73 ns per call in C. When enabling this option, the two
> > languages become equally fast, and disassembly confirms the exact same
> > machine code is used (in particular there is no call to
> > rust_helper_refcount_inc). Benchmarking Binder also results in an
> > improvement from this change.
> >
> > This patch is complementary to:
> > https://lore.kernel.org/all/20251202-define-rust-helper-v1-0-a2e13cbc17a6@google.com/
> >
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> 
> I get the following modpost errors when building with this applied on top
> of v6.19-rc4:
> 
> ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/drm/nova/nova.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [drivers/gpu/nova-core/nova_core.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [drivers/block/rnull/rnull_mod.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_minimal.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_misc_device.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_print.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_dma.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_pci.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_platform.ko] undefined!
> ERROR: modpost: "__SCK__WARN_trap" [samples/rust/rust_driver_faux.ko] undefined!

Looks like it used the wrong static_call_mod() version, was MODULE
defined?